Until recently, repair and retrofit options were limited and costly. The usual solution for a sick building is to add steel beams underneath sagging floors and a concrete topping. But while this method may achieve its objective, the cost, disruption to utility services, and unsightly steel work may be unacceptable to building occupants and owners. Rather than face these obstacles, some owners opt to tear down and start from scratch. But now there is an alternative to these methods. External post-tensioning (EPT) has been successfully applied to repair or retrofit about 30 concrete structures in the United States.
With EPT, tendons are used on the outside of a steel, concrete, or wood structure to bolster its strength. Cores are drilled through the concrete walls and columns in order to attach the tendons. The tendons are then run underneath the member that requires strengthening and steel saddles are placed between the tendon and floor at precise locations. The tendons are then tensioned, and they apply a vertical, upward force to the bottom of the floor. This method allows for the application of large upward forces at practically any location in a sagging floor. It also increases shear strength and helps eliminate further deflection.